We have examined the effect of T‐ansforming Growth Factor (TGF) beta on proliferation of L6 and embryonic porcine myogenic cells. Proliferation of L6 cells was suppressed by both TGF beta‐1 and TGF beta‐2 in a dose‐dependent manner. Half‐maximal suppression of proliferation occurred at.036 ng TGF beta‐1/ml and.06 ng TGF beta‐2/ml. Maximal inhibition (60% suppression of proliferation for TGF beta‐1 and 52% for TGF beta‐2) occurred between.1 and.3 ng/ml for each growth factor. Suppression of proliferation was completely abolished in the presence of an anti‐TGF beta antibody that inhibited the biological activity of TGF beta‐1 and TGF beta‐2. When we evaluated the effect of TGF beta‐1 on proliferation of embryonic porcine myogenic cells we obtained results which were very similar to those obtained for L6 cells. Insulin‐like growth factor (IFG)‐I stimulated proliferation of L6 cells in a dose‐dependent manner in serum‐free, defined medium. However as little as.02 ng TGF beta‐1/ml detectably suppressed this stimulation and.3 ng TGF beta‐1/ml caused a 60% reduction in cell number in cultures treated with 30 ng IGF‐1/ml. Thus TGF beta‐1 significantly suppressed IGF‐l‐stimulated proliferation of L6 cells.